Said glass substrate plates or flat panel displays (FPD) are used for producing individual LCD displays, which are cut out of the glass substrate plates. The glass substrate plates must for this purpose be treated in a treatment process, which implies material deposition, irradiation, etching, etc. and to this end they must be introduced into and removed from a process station again.
After treatment, the glass substrate plates have a process side to which the LCD is applied. They also have a nonprocess side. To avoid quality deterioration, during production and transportation contacts necessary as a result of gripping or depositing the glass substrate plates must only occur on the nonprocess side, but never on the process side. Thus, the glass plates must always be gripped on one side.
During the production process the glass substrate plates are stored in magazines or so-called cassettes and in this way are conveyed to the process station. For the storage of the glass substrate plates in the cassettes, they are stored on the nonprocess side in horizontal orientation in the cassette. As a function of the glass substrate size, there can be up to 50 glass plates in a cassette. For this purpose the glass substrate plates rest on retaining arms within the cassette.
For the production process the glass substrate plates must be removed from the cassette and deposited at process stations (prior to process treatment) or received (following process treatment). The transfer interface at the process station is i.a. defined as a roller conveyor on which can be deposited a glass substrate plate, once again on the nonprocess side. Besides process stations with feed units, there are also process stations in which the glass substrate plates are placed in a direct manner and consequently there is no need for a feed unit. On conclusion of the given process, the glass substrate plate must once again be removed from the process station (frequently via the same interface) and placed in the cassette.
Handling from or into cassettes and process stations nowadays takes place by means of lifting mechanisms, which are located directly at the transfer interface of the process stations or with the aid of robots specifically developed for this purpose and having corresponding gripping tools. Gripping (receiving and depositing) of the glass substrate plates in the latter case takes place with the aid of a gripper, which is equipped with fork elements (fork grippers). The arrangement of the forks is chosen in such a way that they can be positioned between the retaining arms within the cassette or between the rollers of the feed units. The forks are then in each case located below the glass substrate plate to be received (nonprocess side).
In order to prevent a slipping of the glass substrate plates on the gripping forks, they are generally equipped with vacuum suction pads and therefore ensure a nonpositive fixing of the glass substrate plates.
In the prior art the basic handling process can be subdivided in the following way:    1. Introduction of the fork gripper into the cassette.    2. Raising the fork gripper and therefore the untreated glass substrate plate from the retaining arms within the cassette.    3. Fixing the untreated glass substrate plate to the fork gripper (e.g. by suction, clamping, etc.).    4. Removing the fork gripper with the glass substrate plate from the cassette.    5. Transporting the glass substrate plate to the process station.    6. Positioning the glass substrate plate on the feed unit or directly in the process station.    7. Releasing the fixing and depot of the glass substrate plate (fork gripper moves between the rollers or depot elements of the feed unit or process station).    8. After the end of the process raising the fork gripper and therefore the treated glass substrate plate.    9. Fixing the untreated glass substrate plate to the fork gripper (e.g. by suction, clamping, etc.).    10. Transporting the treated glass substrate plate to the cassette.    11. Insertion in the cassette.    12. Positioning the treated glass substrate plate above the retaining arms.    13. Releasing the fixing and depot of the glass substrate plate (fork gripper inserted between the retaining arms of the cassette).    14. Removing the fork gripper from the cassette and positioning the fork gripper for receiving the next untreated glass substrate plate.
As stated hereinbefore, in connection with the handling process the presently used handling systems only have the possibility of in each case transporting one glass substrate plate. This means that the robot for handling an individual glass substrate plate must be moved from the cassette to the process station feed unit and back again.
It is known to construct the grippers as double grippers, which can consequently receive two glass substrate plates. This type of gripper or handling is used in order to receive an untreated glass plate from the cassette and at the process station receive the treated glass substrate plate and without any further movement on the part of the handling device to deposit there again the next, untreated glass substrate plate. Thus, the double gripper is only used at the interface to the process station, so that it is possible to deposit an already gripped, untreated glass substrate plate on the feed unit or directly in the process station after removing the treated glass substrate plate. This economizes movements on the part of the robot and minimizes the cycle time.
As fork grippers the double grippers are superimposed and can therefore generally be moved horizontally independently of one another by means of a straight line gear. The lifting movement for depositing or receiving the glass substrate plates in the cassette or process station is, however, coupled, which leads to the disadvantage that the depositing or receiving of glass substrate plates in the cassette cannot take place simultaneously through both gripper forks and instead these two handling processes must be performed sequentially (handling process 1: inserting gripper 1 in the cassette and depositing the treated glass substrate plate; handling process 2: inserting gripper 2 in the cassette and receiving an untreated glass substrate plate). If both grippers operated simultaneously in the cassette, a treated glass substrate plate deposited by gripper 1 in the cassette would be received again when an untreated glass substrate plate was received through gripper 2. Thus, with such a double gripper solution such as is provided by the prior art, the productivity of an installation can be increased by avoiding unnecessary transportation procedures, but the sequential handling process at the cassette still takes up much time.
There is also an increased contamination risk of the lower glass substrate plate emanating from the robot arm which receives the upper fork gripper. This moves directly above the glass substrate plate. Through bearing friction on the robot arm joints particles can be detached, which can be deposited on the process side of the glass substrate plate. As great importance is attached to the requirement of particle freedom when handling glass substrate plates (clean room), these represent not inconsiderable risk factors.
Such a double gripper is e.g. described in JP 2005-230941 A. In the prior art solutions only a single fork gripper can be introduced into the cassette or process station in order to deposit or receive a glass substrate plate.
On the basis of this prior art the problem of the invention, whilst avoiding the aforementioned disadvantages, is to provide an apparatus and a method for transferring and accepting glass substrate plates, by means of which gripper movements and transportation processes are minimized and consequently it is possible to reduce the cycle time and increase productivity.